The following reference publications are of interest in regard to the present invention, a method for processing organic sludges.
"Land Disposal--What's the Realistic View", June, 1974, by authors H. L. Michel, P. H. Gilbert, and H. K. Creed, published in Water and Wastes Engineering, June, 1974; "Utilization of Animal Manures and Sewage Sludges in Food and Fiber Production", Report 41, February, 1975; and "Application of Sewage Sludge to Croplands; Appraisals of Potential Hazards of Heavy Metals to Plants and Animals", Report No. 64, November, 1976, both published by the Council for Agricultural Science and Technology (CAST), Iowa State University; and, in regard to prior art solar ponds, "Energy Production by Microbial Photosynthesis", John R. Benemann, et al. Nature, Vol. 268, No. 5615, pp 19-23, July 7, 1977.
The potential for the recovery of resource values from sludge from conventional sewage treatment facilities is evident from the data published. Expressed in volumes per 100,000 persons, the average dryweight volume of sludge produced is about 11.5 tons per day containing about 65% organic solids of potential value as fuel or as agricultural fertilizer and soil amendment materials.
The PBQ&D, Inc., and CAST studies referred to above also provide data as to the substantial amounts of heavy metals in sewage sludges. For example, the following concentrations are noted:
______________________________________ Element Range mg/kg - dry weight basis ______________________________________ Zinc 500 to 50,000 Copper 250 to 17,000 Nickel 25 to 8,000 Cadmium 5 to 2,000 ______________________________________
The potentials for resource recovery and recycling of metal values is of special interest. U.S. Geological Survey Circular 784 (1978) describes a special situation in which gold and silver are present in sewage sludge incinerator ash in quantitites providing incentives for special treatment to recover precision metals.
Land disposal of primary and mixed sludge is currently practiced, but is costly as most of the total weight is water and the potentials for heavy metals pollution require sometimes costly mitigating measures. Incineration is difficult because of the water content and it is costly due to supplemental fuel usually required to support combustion. Discharge of sludge into public waterways and the oceans is not an acceptable alternative in the opinion of many environmental experts.
The PBQ&D, Inc. reference study also documents the presence of substantial amounts of heavy metal residuals in treated wastewater effluents. The presence of such toxins limits the use of such effluents for agricultural irrigation. A tertiary treatment is often required prior to discharge to public waterways.